1. Field of the Invention
The invention relates to multiple-phase switched reluctance machines (SRMs) and, more specifically, to two-phase self-starting. SRMs. Additionally, the invention relates to SRMs and permanent magnet brushless direct current (dc) machines (PMBDCMs) and, more specifically, to reducing vibration and acoustical noise in SRMs and PMBDCMs.
2. Description of Related Art
Related art two-phase switched reluctance machines (TPSRMs) do not produce electromagnetic torque at certain rotor positions, i.e., when the rotor poles are midway between the stator poles. At such rotor positions, the electromagnetic torque that can be produced is zero for both phases. Therefore, exciting one or both phase windings does not help in moving the rotor from its stand-still position. Because of this, these machines are not self-starting from stand-still. This particular problem of zero torque does not arise in the case of SRMs with more than two phase windings.
The natural question that arises is why not use SRMs having three or more phases and completely do away with two-phase machines. The answer lies in the fact that the TPSRM requires fewer power devices, such as controllable power switches and power diodes, in its power converter as compared to machines with three or more phases. Using fewer power devices in the power converter lends itself to a large saving in power converter cost. This cost saving in the power converter leads to a greater cost reduction of the total motor drive system.
Moreover, a reduction in the number of power devices provides additional benefits. For example, a power converter using fewer devices can employ a smaller heat sink for the thermal management of the power devices, fewer gate drive circuits, fewer gating power supplies, and fewer components in the drive circuits and gating power supplies. Additionally, such a power converter can employ fewer pulse width modulation (PWM) channels, due to the reduction of power devices, and has a lower control burden, as only two phases have to be controlled as opposed to three or more phases. All of these factors contribute to reducing the number of components and simplifying the microcontroller that is used to control the power converter, resulting in a large reduction in cost of the motor drive system. The principle and operation of the SRM is explained in many publications and in the book “Switched Reluctance Motor Drives,” R. Krishnan, CRC Press, June 2001, which is hereby incorporated by reference. For brevity, all of this description will not be provided herein, but may be obtained by reference to the identified book.
FIG. 1 illustrates a related art TPSRM. TPSRM 100 has a stator 101 with four salient stator poles 102 and a rotor 103 with six salient rotor poles 104. Rotor 103 is mounted on a shaft 105 for rotation within stator 101, about the axis of shaft 105. Stator poles 102 are wound with concentric stator windings 106-109. The stator windings 106, 107 and 108, 109, respectively, on the diametrically opposite stator poles 102 are connected usually in series, though sometimes in parallel, and the two serially connected coils constitute a phase winding, commonly referred to as stator phase. Therefore, a four-pole stator 101 will have two phase windings or two stator phases.
A rotor normally has two poles in a TPSRM. For this description, though, TPSRM 100 has a six-pole rotor 103. This combination is referred to as a 4/6 machine, the first number denoting the number of stator poles 102 and the second number denoting the number of rotor poles 104.
FIG. 2 illustrates a graph of the flux linkages for the TPSRM illustrated by FIG. 1 plotted as a function of rotor position, using a fixed stator excitation in the phase windings. FIG. 3 illustrates a graph of the electromagnetic torque for the TPSRM illustrated by FIG. 1 plotted as a function of its rotor position, using a fixed stator excitation in the phase windings. The flux linkages are defined as the product of lines of flux that link the number of winding turns in a phase winding and have been computed here using a two-dimensional finite element analysis method. The electromagnetic torque is the torque generated in the air gap of the machine, due to the flux and current in the machine, and is computed from the flux linkages versus rotor position characteristic for a given excitation current in the phase windings.
Because of the symmetry of rotor poles 104 with respect to a set of stator poles 102, flux linkages 201 and 202 and, hence, torque characteristics 301 and 302 are symmetric, resulting in instances where torques 301 and 302 are simultaneously zero for both phases. These instances of zero torque make it difficult to start TPSRM 100, if the stand-still or start-up rotor positions correspond to them. Therefore, TPSRM 100 is inherently incapable of self-starting in both directions of rotation, if provision is not made to eliminate these simultaneous zero-torque instances.
Two-phase SRMs have been in research since 1969. Most of these TPSRMs have four stator poles and two rotor poles. In order to have any directional starting capability, these machines have had their rotor poles shaped. Such rotor pole shaping is described by Byrne et al., in U.S. Pat. No. 3,956,678.
Byrne discloses rotor pole shaping taking the form of a very extended pole arc, exceeding 100 degrees, which is unusual for an SRM and is denoted in literature as an arcuate pole. Byrne also discloses populating half of the distal end portion of the rotor poles with trapezoidal slots, for the purpose of producing a flux that is linear with angular position. This feature makes the slope of the flux with respect to the rotor position a constant, resulting in constant torque. When the slotted portion of the rotor is in alignment with the stator pole, only half the flux flows as compared to a case where the unslotted portion of the rotor is aligned with the stator pole. This is the reason why the rotor pole has an arc of greater than 100 degrees.
U.S. Pat. Nos. 5,747,962, 5,844,343, and 6,005,321 disclose another approach to providing TPSRMs with a self-starting capability. This approach is to have rotor poles with two steps, known as stepped rotor pole faces, so that the low or high air gap part of the rotor pole pair is close to one or the other phase's stator poles. This ensures that there is a reluctance variation available for producing electromagnetic torque. The disadvantage of such an approach is that wider rotor poles are employed, giving way to low torque density and a nonlinear relationship between the flux and rotor position. Due to the nonlinear relationship between the flux and rotor position, the TPSRM has a large torque ripple and a varying torque constant, thus making control of the machine difficult.
U.S. Pat. No. 4,698,537 discloses another way to create overlapping torque in TPSRMs, which involves using pole shoes. The pole shoes provide a path for fringing flux, when both phases are excited simultaneously. The fringing flux produces starting torque when the rotor poles are completely aligned with a set of stator poles. This may be viewed as the torque due to mutual flux linkages. The pole shoes accentuate the mutual flux linkages, since the pole shoes overarch a set of stator poles and come close to the other set of poles in terms of their reach, thus facilitating a mutual coupling through a flux path. The rotor slotting is used here to provide the linear flux to rotor position relationship and not for augmenting the starting torque. Also, this technique is intended for machines having a two-to-one ratio between stator and rotor poles.
U.S. Pat. Nos. 5,747,962, 5,844,343, and 6,005,321 also disclose various other approaches for providing a self-starting capability, using a stepped air-gap rotor with two- or even three-phase SRMs. The second and third stator windings are called auxiliary windings. They are mainly used to augment the regular torque, or used in starting.
U.S. Pat. No. 5,747,962 discloses that a simultaneous excitation of the two phases produces enhanced torque, both at starting and running. A control system to achieve the simultaneous excitation is presented, though this control system does not provide current profiling due to its perceived complexity.
U.S. Pat. No. 5,844,343 discloses that an auxiliary winding is selectively utilized for starting or augmenting the torque. Stator pole shifting is also used for starting purposes.
U.S. Pat. No. 6,005,321 discloses that two auxiliary windings are used to generate the starting torque. In all these approaches, three or more controllable switches are needed to realize variable speed operation of the SRM. Moreover, these stepped air gap approaches do not significantly alter the requirements of a regular two-phase SRM.
U.S. Pat. No. 6,028,385 discloses another approach whereby the SRM has eight stator poles and four rotor poles, and one pair of rotor poles is wider than the other. This configuration provides non-zero torque at every possible rotor position, when both phases are used to generate torque. This approach seems to exploit the earlier approaches of wider rotor poles, but only for one set of rotor poles rather than for all the rotor poles.
U.S. Pat. Nos. 6,046,568 and 6,051,903, issued to Pengov, disclose similar approaches whereby one set of rotor poles is wider than the other set. Pengov discloses configurations whereby both pole sets align with respective stator poles of a first phase, while at the same time the wider rotor pole set communicates with the stator poles of a second phase. These configurations provide a favorable torque generation aspect to the next or successive phase. The concept has been extended to three- and four-phase SRMs. The disadvantage of this approach is that one set of rotor poles has to be twice or more as wide as the other set of rotor poles, resulting in high rotor iron volume and weight. Thus, these approaches reduce the power density of the machine. Furthermore, these approaches complicate the control system, due to the many different modes that exist in the fluxing of the machine phases.
Some of the above described related art approaches to providing a self-starting TPSRM have an unevenness in the mechanical structure of the rotor, resulting in a non-uniform air gap when the stator and rotor poles align. These approaches are prone to normally-induced forces and, hence, may have a slightly higher acoustic noise. This is yet to be proved, but can be inferred from the electromagnetic structure and the slope of the inductance curves of the machines. Therefore, a better and improved method of producing an electromagnetic torque at all rotor positions is desirable.
All reference material cited herein is hereby incorporated into this disclosure by reference.